The invention relates to electron discharge devices and more particularly to image and X-ray intensifier tubes.
Image intensifier tubes are well known in the art of electron discharge devices. Such devices, are for example, described in the book entitled Photoelectronic Imaging Devices, Volume 2, edited by L. M. Biberman and S. Nudelman, on pp. 119-165, Plenum Press, N.Y., N.Y. (1971) and in an article entitled "Microchannel Plates Advance Night-Viewing Technology"by Martin J. Needham, in Electronics magazine, Sept. 27, 1973, McGraw-Hill, N.Y., N.Y.
Image intensifier tubes are particularly useful for producing on an output screen of the tube an amplified visible image of a radiation pattern which impinges upon an input screen of the same tube. An image intensifier tube generally includes three functional parts: (1) an input screen including a cathode which emits electrons in a density pattern corresponding to the incident radiation pattern, (2) an electron lens system which accelerates the emitted electrons and forms an electron-image of the incident radiation pattern, and (3) an output screen including a phosphor screen placed in the plane of the electron image for converting the electron-image into an output image. Image intensification occurs as a consequence of electrons, which are emitted by the cathode, striking the phosphor screen after being accelerated by the electron lens system at a high voltage (e.g. 8-16 K.V.) with respect to the potential applied to the cathode.
When the output screen of a prior art image intensifier tube is grounded and the tube is surrounded by a grounded shield in the conventional manner generally employed during operation of image tubes, the output image of the operative tube is degraded by spurious light flashes or background signals. However, when the cathode of the input screen is grounded relative to the output screen, the problem is minimized. Unfortunately, such tubes must often be operated in combination with other devices (e.g. silicon intensifier tubes having one or more stages of image intensification, or cascaded image intensifier tubes) where the output screen is substantially at ground potential and where the input screen is at a relatively high negative potential relative thereto, (e.g. -21 K.V.). When the tube is operated in this fashion, portions of the interior wall surfaces of glass or ceramic envelope insulating and support members of the tube are charged positively toward the output screen potential. The charged interior wall surfaces of the envelope attract stray or divergent electrons to the wall causing secondary electrons to be emitted thereby further increasing the positive potential or charge along the interior surface of the envelope. The charged wall surface of the envelope tends to modify the electron accelerating fields within the tube and to divert more electrons to impinge upon the same wall surface portions. As a consequence of the above-noted electron bombardment of the interior envelope wall surface, positive ions are released which are accelerated to impinge upon the cathode of the input screen. The bombardment of the cathode with ions causes the undesirable spurious background electron emission previously described and seriously impairs the signal to noise ratio of the tube.